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19.5.2.5. Polygon函数

• Area(poly)

  以双精度数值形式返回Polygon值poly的面积，根据在其空间参考系中的测量值。

  mysql> SET @poly = 'Polygon((0 0,0 3,3 0,0 0),(1 1,1 2,2 1,1 1))';
  mysql> SELECT Area(GeomFromText(@poly));
  +---------------------------+
  | Area(GeomFromText(@poly)) |
  +---------------------------+
  |                         4 |
  +---------------------------+
  

• ExteriorRing(poly)

  以LineString形式返回Polygon值poly的外环。

  mysql> SET @poly =
      -> 'Polygon((0 0,0 3,3 3,3 0,0 0),(1 1,1 2,2 2,2 1,1 1))';
  mysql> SELECT AsText(ExteriorRing(GeomFromText(@poly)));
  +-------------------------------------------+
  | AsText(ExteriorRing(GeomFromText(@poly))) |
  +-------------------------------------------+
  | LINESTRING(0 0,0 3,3 3,3 0,0 0)           |
  +-------------------------------------------+
  

• InteriorRingN(poly,n)

  以LineString形式返回Polygon值poly的第n个内环。环编号从1开始。

  mysql> SET @poly =
      -> 'Polygon((0 0,0 3,3 3,3 0,0 0),(1 1,1 2,2 2,2 1,1 1))';
  mysql> SELECT AsText(InteriorRingN(GeomFromText(@poly),1));
  +----------------------------------------------+
  | AsText(InteriorRingN(GeomFromText(@poly),1)) |
  +----------------------------------------------+
  | LINESTRING(1 1,1 2,2 2,2 1,1 1)              |
  +----------------------------------------------+
  

• NumInteriorRings(poly)

  返回Polygon值poly的内环的数目。

  mysql> SET @poly =
      -> 'Polygon((0 0,0 3,3 3,3 0,0 0),(1 1,1 2,2 2,2 1,1 1))';
  mysql> SELECT NumInteriorRings(GeomFromText(@poly));
  +---------------------------------------+
  | NumInteriorRings(GeomFromText(@poly)) |
  +---------------------------------------+
  |                                     1 |
  +---------------------------------------+